The demand for improved print quality in xerographic reproduction is increasing, especially with the advent of color. Some of the print quality issues such as the defect level of the charge deficient spots ("CDS") and the print defects caused by bias charge roll ("BCR") leakage, are strongly dependent on the quality of the charge blocking layer. Conventional materials used for the blocking layer have been problematic. In certain situations, a thicker blocking layer is desirable, but the thickness of the material used for the blocking layer is limited by the inefficient transport of the photoinjected electrons from the generator layer to the substrate. Another problem is posed by a blocking layer that is too thin: incomplete coverage of the substrate due to wetting problems on localized unclean substrate surface areas. These pin holes can then produce CDS and BCR leakage breakdown. A thicker blocking layer can be produced by dispersing titanium dioxide particles into a binder, which can allow the transport of photogenerated electrons and may eliminate any pin holes due to incomplete coverage. In certain situations, a high concentration of titanium dioxide in the blocking layer is desirable. However, the dispersion quality such as particle size distribution may be significantly worse at a high titanium dioxide concentration. Poor dispersions often cause coating defects such as streak and coating non-uniformity. The dispersion quality of titanium dioxide depends on the binder and solvent employed. Conventional binders and solvents may be unsuitable at a high concentration of the titanium dioxide. In addition, some conventional binders are soluble in the solutions coated onto the substrate after the blocking layer such as the solutions for the charge generating layer and the charge transport layer. Such a solubility allows intermixing of layers that results in electrical and print quality problems. Thus, there is a need, which the present invention addresses, for new binders for the blocking layer of a photoreceptor that minimize or eliminate the problems of conventional binders described herein.
The phrases "charge blocking layer" and "blocking layer" are generally used interchangeably with the phrase "undercoat layer."
Yashiki, U.S. Pat. No. 4,579,801 discloses an electrophotographic photosensitive member having a phenolic resin layer formed from a resol coat, between a substrate and a photosensitive layer.
Conventional photoreceptors and their materials are also dislosed in Katayama et al., U.S. Pat. No. 5,489,496; and Suzuki, U.S. Pat. No. 5,965,311.
Huoy-Jen Yuh et al., U.S. application Ser. No. 09/416,840, discloses a charge blocking layer including a binder, a plurality of grain shaped n-type particles and a plurality of needle shaped n-type particles.